This avoids the possibility of initializing globals twice in Chromium.
Now we also never initialize variables redundantly in case we're
writing HLSL output. This was already the intent of the code before,
but a mistake had slipped in and the code didn't actually check the
output type properly.

This also simplifies DeferGlobalInitializers by running it after
SeparateDeclarations.

BUG=chromium:735497
TEST=WebGL conformance tests

Change-Id: I95036a24ac8cf18113755510376a2fca286b3ee6
Reviewed-on: https://chromium-review.googlesource.com/771555Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>